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Astronomers say it was the first stars and galaxies, rather than black holes or elemental particle decay, which gave us the universe we have today.

Looking at the latest research, a team of scientists including Dr Brant Robertson from the California Institute of Technology has concluded this first generation of stars provided enough energy to reionise the universe, making it transparent, rather than a thick fog of neutral hydrogen gas.

Ionisation is the process of converting an atom-like hydrogen into an ion by removing a charged particle such as an electron.

In their review paper in the journal Nature, Robertson and colleagues say recent observations - made with a powerful new camera aboard the Earth orbiting Hubble Space Telescope - have allowed scientists to see the first generations of stars and galaxies.

This instrument, called the Wide Field Camera 3, is a panoramic imager that includes an infrared detector that lets astronomers detect extreme redshift galaxies, going back to when the universe was less than a billion years old.

The most ancient history of all

The universe was born 13.7 billion years ago in a searing soup of quark gluon plasma, slowly cooling and expanding over the eons.

About 370 thousand years after the Big Bang, the universe had cooled enough for the first electrons and protons to come together and form neutral hydrogen, giving off the cosmic microwave background radiation which we still see today at minus 270 degrees Celcius, just 3 degrees above absolute zero.

This was a time cosmologists call the dark ages of the universe which lasted until the first stars formed from over dense clouds of hydrogen gas that cooled and collapsed in early cosmic structures.

These first generations of stars were far more massive than those we see today, and gave off enormous amounts of high-energy radiation.

Reionisation

Robertson and colleagues found data from Hubble and other observations show that there were enough of these first generation star-rich galaxies at redshifts near 7 (which indicates that they are more than 13 billion years old), to flood the cosmos with high energy ultraviolet photons and reionise all the neutral hydrogen in the intergalactic medium.

Dr Chris Lidman from the Australian Astronomical Observatory says: "There was always a question of whether it was stars or high energy photons from material accreting onto black holes or the decay of elementary particles".

"You take ultra-deep field images at extreme redshifts, you count the galaxies and work out how many young stars they contain by how bright they are", he says.

"From that you can calculate how many high-energy photons are being produced."

While this method indicates that there were enough high-energy photons to reionise the universe, he says there are still some details to be worked out.

"These photons need to be able to escape the galaxies themselves before being able to reionise the universe. It's what's known as the escape fraction of these high energy photons."

"If the escape fraction is too low, then there won't be enough to cause reionisation, so there are still some uncertainties that need to be resolved."

Lidman says reionisation probably started about half a billion years after the Big Bang and was mostly complete by the time the universe was a billion years old.